Screws are often used for a removable yet also sufficiently firm attachment of implants. Depending on the application, a rigid-angle positioning between the implant and the bone screw is sufficient; in other cases, however, a variable-angle (polyaxial) arrangement of the bone screws is required. Such a polyaxially mounted bone screw for a fixation apparatus is known in its basic features from EP 0 614 649 B1. The bone screw is led through a support sleeve, which has a screw-on nut for the securing. In order to attach the rod of an implant, a transverse borehole is provided in the support sleeve, through which the rod can be passed in an intermediate region between the nut and the bone screw. By tightening the nut, the rod is braced against the support sleeve. Large tightening forces can twist the support sleeve, which endangers the fastening safety.
In a further developed design, as is known from U.S. Pat. No. 7,641,674 B2, a pressure screw screwed into the rear end of the support sleeve acts on a tensioning element by a support rod of the implant that is inserted through a transverse bore, which has at its front end a dome-shaped recess to receive a spherically thickened screw head of the bone screw. The tensioning element can move lengthwise and is pressed by the force of the pressure screw against a snap ring arranged at the front end of the support sleeve. Thus, it forms an abutment for the pressing force applied by the pressure screw. This design limits the pressing forces which can be applied, and thus the clamping forces needed to achieve an angle stability.